In certain instances, anions (e.g. sulfate ions) are removed from liquid samples by passing through a packed bed of cation exchange resin with an exchangeable polyvalent cation (e.g. in barium-form) with which the sulfate ion forms a precipitate. Such removal is performed in a pretreatment step prior to the separation and detection of other anions in a sample to prevent adverse effects on performance. The barium ion is displaced from the ion exchange resin by other cations in the sample so that precipitation can occur. However, since barium is a divalent cation, the cation exchange resin has much higher selectivity for barium than the monovalent ions, such as sodium, present in abundance in a typical sample. Polyvalent cations are not normally present in a sample at a high enough concentration to displace sufficient barium for complete sulfate removal. Therefore, the effectiveness of removal of sulfate from liquid samples by precipitation in barium-form media depends on the composition of the sample matrix. This leads to unpredictable results for samples of unknown cation composition. This same problem applies to the removal of cations (e.g. barium ions) using a polyvalent anion exchange bed.
It is an object of the invention to improve the removal of selected anions from a liquid sample, using a packed bed.